1. Field of the Invention:
The present invention relates to an apparatus for manufacturing curved glass sheets, and more particularly to a glass sheet conveying mechanism in such an apparatus for conveying glass sheets softened by the application of heat from a heating station to a bending station.
2. Description of the Relevant Art:
Toughened shaped or bent glass sheets are widely used as automotive windshields. Such a glass sheet is required to be accurately curved to the shape of an outer profile of an automobile and the shape of a window frame in which the glass sheet is to be mounted. The shaped glass sheet should not contain any optical defects which would otherwise interfere with the clear vision of the automobile driver through the glass sheet.
One of the most commonly practiced processes of manufacturing curved glass sheets starts with the step of heating a flat glass sheet in a heating furnace up to a temperature where glass is deformed, i.e., the softening point of the glass. Then, the heated glass sheet is fed from the heating furnace to a mold assembly by a suitable conveyor means, and then curved to a desired curvature by the mold assembly. After the glass sheet has been bent to shape, it is finally fed to a cooling station by the conveyor means and quenched or annealed by the application of a suitable coolant. The glass sheet is first heated and then quenched for the purpose of making the glass sheet more resistant to breakage, and of causing the glass sheet to fracture into relatively small, relatively smoothly surfaced fragments that are less injurious to passengers than large, jagged fragments.
One typical form of the glass sheet conveyor means as referred to above is disclosed in U.S. Pat. No. 4,252,552 issued to Robert G. Frank on Feb. 24, 1981. The disclosed conveyor means includes a number of conveyor rolls spaced at equal intervals in the direction in which glass sheets are fed along and having axes extending in a direction normal to that direction. A glass sheet conveyed out of a heating furnace by conveyor rolls disposed therein is transferred onto conveyor rolls in a bending station, and then into a mold assembly where the glass sheet is stopped. When the glass sheet is stopped, a lower mold below the conveyor rolls is lifted by a raising and lowering means until the upper surface of the lower mold is held against the lower surface of the glass sheet. The lower mold has a downwardly concave shaping surface with a plurality of grooves extending transversely across the direction in which the glass sheet is conveyed, the grooves being spaced at the same intervals as those of the conveyor rolls. When the lower mold is further raised, the conveyor rolls are retracted into the grooves for thereby supporting the glass sheet only on the upper surface of the lower mold, whereupon the glass sheet is shaped or curved into a configuration complementary to the curved shape of the shaping surface of the lower mold. Then, an upper mold is lowered to attract the glass sheet against its lower surface under vacuum, and then is elevated to lift the glass sheet. Now, a ring mold is introduced between the upper and lower molds, and the vacuum in the upper mold is removed by an evacuation device to allow the glass sheet to be transferred onto the ring mold. The ring mold has a greater curvature than that of the upper mold for curving the marginal edge of the glass sheet to a larger curvature than that of the other area of the glass sheet. Identical or similar conveyor rolls are also disclosed in U.S. Pat. Nos. 3,455,672, 3,782,916, 4,197,108, 4,272,274, 4,285,715, 4,430,110, and 4,589,901.
In each of the conventional apparatus disclosed in the above patents, the lower mold has a plurality of grooves defined therein. When the lower mold receives a glass sheet from conveyor rolls, the lower mold is raised or the conveyor rolls are lowered to retract the conveyor rolls into the respective grooves. However, the disclosed apparatus for manufacturing curved glass sheets are disadvantageous for the following reasons: The lower mold with such plural grooves defined therein is complex in construction, and cannot easily be manufactured. Since the upper surface of the lower mold, which serves as the shaping surface, is only partly used for shaping the glass sheet, certain difficulty is experienced in shaping the glass sheet to desired curvature. Moreover, the efficiency of cooling the glass sheet is poor as holes for ejecting cooling air cannot be defined at uniform intervals in the upper surface of the lower mold. In addition, the conveyor rolls, once they have received a glass sheet from a heating furnace, cannot be corrected in their positions. Therefore, if the glass sheet has inaccurately been transferred from the heating furnace into position on the conveyor rolls, the conveyor rolls cannot convey the glass sheet properly onto the upper surface of the lower mold.